Many optical functions require the ability to change the phase or time delay in an optical path. Using such phase or time delays in interferometric structures allows modulation of the intensity of light beams, switching of light beams between different paths, and control of spectral filtering operations.
One common approach to changing the phase or time delay in an optical path is to change the refractive index of a material in the path. Unfortunately, many such methods can only make very small changes in refractive index, so relatively long paths through the materials are required and only small changes in phase or time delays can typically be created. Materials that offer large changes in refractive index often also have relatively large optical absorption under some of the desired operating conditions, restricting their usefulness. Large path length changes can be made by mechanically moving a reflector. One common method is to use a retroreflector on a translation stage. In such a case, the total optical path length in a beam that travels to the reflector and back again can be changed by moving the reflector. Unfortunately, it is difficult to make a version of such a device that can be conveniently integrated using waveguide paths without being able to stretch a waveguide material, and it is difficult in practice to stretch materials and waveguide structures, such as optical fibers, by large amounts.